Schedule III - Real Estate and Accumulated Depreciation and Amortization - Summary (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
SEC Schedule III, Real Estate and Accumulated Depreciation Disclosure [Abstract]
Total cost basis for federal income tax purposes	$ 4,600,000
Reconciliation of real estate owned:
Balance at the beginning of year	3,671,560	$ 3,812,294	$ 3,564,466
Additions during year	478,717	210,143	492,437
Properties sold during year	(332,670)	(282,143)	(212,771)
Properties impaired during the year	(11,306)	(65,426)	(31,741)
Other reclassifications	(16,590)	(3,308)	(97)
Balance at end of year	3,789,711	3,671,560	3,812,294
Reconciliation of accumulated depreciation and amortization:
Balance at the beginning of year	795,486	775,617	738,068
Depreciation and amortization expense	124,618	119,156	122,057
Accumulated depreciation and amortization of properties sold, impaired and held for sale during year	(106,268)	(98,698)	(84,508)
Other reclassifications	(1,629)	(589)	0
Balance at end of year	$ 812,207	$ 795,486	$ 775,617